1. Field of Invention
The present invention relates to a constant-voltage circuit, a semiconductor apparatus including the constant-voltage circuit, a method of controlling the constant-voltage circuit, and more particularly to a constant-voltage circuit including a soft-start function capable of reducing time required for starting a semiconductor apparatus including the constant-voltage circuit, and a method of controlling the constant-voltage circuit.
2. Discussion of the Background
Recently, in consideration of an environmental issue, there has been demand for electronic equipment consuming less electricity. Such an energy saving trend is particularly noticeable in a field of battery-driven electronic equipment. An important point in a general way of saving power is to achieve a reduction in electricity consumed by electronic equipment, and an improvement in efficiency of a power supply circuit to cut unnecessary consumption of electricity. A general method of reducing electricity consumption of electronic equipment is to keep the electronic equipment in a stand-by state while the electronic equipment is not operating so that operation of a circuit therein is stopped to cut electricity consumption. However, bringing the electronic equipment into the stand-by state may not achieve satisfactory electricity savings when the power supply circuit is not efficient.
General power supply circuits used in electronic equipment include a switching regulator and a series regulator.
Although the switching regulator is highly efficient with a rated load, the switching regulator has a large output voltage ripple and a large operating noise, and internally consumes a relatively large amount of electricity. Therefore, the switching regulator significantly impairs the efficiency under a light load with a small output current. Further, the switching regulator requires a relatively long time for a rise of an output voltage after the power is turned on, and takes a relatively long time for responding to shifts in an input voltage and a load. Therefore, the switching regulator is defective in that the switching regulator has low stability of an output voltage.
On the other hand, although the series regulator has low efficiency with a large amount of load current, as an output transistor consumes a large amount of electricity, the series regulator also has a small output voltage ripple and small operating noise, and a power supply control circuit internally consumes a small amount of electricity. Therefore, the series regulator may achieve higher efficiency than the switching regulator with a small amount of load current. Further, it is easy in the series regulator to reduce amounts of time required for a rise of an output voltage and for responding to shifts in an input voltage and a load. Further, the series regulator has a high stability output voltage.
For the above reason, a background power supply circuit includes both a switching regulator and a series regulator, and selectively uses one of the switching and series regulators according to electricity consumption so as to improve efficiency of a constant-voltage circuit.
FIG. 1 illustrates a diagram of a configuration of a constant-voltage circuit in accordance with an exemplary embodiment of the invention. As shown in FIG. 1, a constant-voltage circuit 101 includes a switching regulator 102, a series regulator 103, and a control circuit unit 104. The switching regulator 102 and the series regulator 103 convert voltages input from a direct current power supply 110 to respective voltages Voa, Vob. The control circuit unit 104 controls a switching operation for switching between the switching regulator 102 and the series regulator 103 so as to selectively operate either the switching regulator 102 or the series regulator 103 according to an external switch control signal Sc. When controlling the switching operation, the control circuit unit 104 first causes both the switching regulator 102 and the series regulator 103 to operate, and then causes a desired one of the switching regulator 102 or series regulator 103 to continue operating.
On the other hand, a general switching regulator includes a soft-start circuit for gradually raising an output voltage so that an excessive current does not flow into a switching transistor on startup. In one example, the soft-start circuit gradually increases a reference voltage; and in another example, the soft-start circuit uses a D/A converter to gradually increase a reference voltage.